It's hard to close the windows after you've left the room

I was in a room at work today that had a sign on the inside of the door saying something like this:

CLOSE WINDOWS

BEFORE LEAVING

THE ROOM

Underlining is the written form of stress on that word (before), and that kind of stress in that position can only make the word contrastive. That is to say, it contrasts before with after and any other word from the appropriate semantic and syntactic class (while, during, etc). It also makes before the focus of the sentence, so that the rest of it seems to be already-known information. The implication is that people have been trying to close the windows after they've left the room, it hasn't worked out well, so now we're being reminded to do it before and not after we leave.

Code-switching contrastive emphasis

I'm watching Salamander. It's a kind of political thriller, lots of running about, photocopying and suchlike. It's also Belgian, and the dialogue is mostly in Flemish. It's a bit complicated to explain the situation, because 'Flemish' is Dutch as spoken in Flanders, but then some varieties (eg West Flemish) can be considered separate languages, and Flemish is a cultural label as well, and... well. Anyway. They're speaking Flemish for most of the time. Every now and then, though, they switch into French, either for a single word, a few words, or some of the characters seem to use French as their preferred language so you get a whole conversation in it. You also sometimes get 'parallel talk', where one person in a dialogue uses Flemish and another uses French.

One such scene took place, in which a dialogue took place between about five people, one of whom was using French. He said 'We have to know who our enemy is', and a Flemish speaker replied 'enemIES', stressing the fact that there are more than one single enemy. This is contrastive stress, and is a nifty way of indicating that you're contrasting something you've said with something the other person either said or implied. The contrast can be lexical/semantic: I want BEANS (not cheese), or it can be grammatical, as you can see here, where singular is contrasted with plural. Normally you need to contrast two things that are similar in some way, like two nouns/foodstuffs/potato fillings.

What's special about this is that the two languages do plural in different ways. French adds an -s in the written language, going from ennemi to ennemis. But in fact, in speech, you won't usually hear that -s, and the only thing to tell you the difference is the 'determiner', an article, demonstrative, etc. In this case it was 'our', which in French is notre for singular and nos for plural. I didn't catch the exact word in Flemish but in Dutch it's vijand for 'enemy' and the plural is marked with an -en: vijanden. The possessive determiner stays the same, and in any case the Flemish speaker didn't repeat 'our'. So we had this:

A: (We have to know who is) notre ennemi. 

B: vijandEN.

The Flemish speaker contrasted a possessive determiner with a totally different morphosyntactic category, a plural inflection, because he was contrasting the feature [number], which is encoded on the determiner in French and the noun suffix in Flemish.